Electrically propelled work machines have been known in one form or another for many years. The continued depletion of fossil fuel reserves, coupled with increasing concerns about certain emissions from internal combustion engines, however, has motivated industry to adopt an increasing number of electrically propelled work machines. Interest in electrical propulsion systems for heavy equipment, in particular, has increased in recent years. In addition to the environmental and economic reasons for the rise in electrically propelled work machines, industry has begun to deliver better and more practical electrical generation and propulsion technologies to the marketplace.
With the advent of superior electrical propulsion system technologies have come new problems. One challenge in particular relates to the interaction of certain electrically propelled work machines with a work material, especially a work material pile. Over many years of experience with traditional work machine designs and operating systems, operators have come to expect certain performance and operating characteristics in particular types of work machines. For example, loader operators who have been trained on loaders having conventional hydraulic or partially hydraulic drive trains, have come to expect certain operating phenomena when loading material from a material pile.
One typical loading operation consists of an operator driving a loader into a material pile, to fill or partially fill the front-end bucket with work material. Once the loader has been driven into the pile, the operator may tilt the bucket back and upward, to lift the work material into the bucket. The operator can then reverse the work machine, and drive the loader with its bucket contents to a dumping site or truck. Substantially simultaneous with driving the loader into the pile, the operator may begin activating the bucket hydraulics. Activation of the bucket hydraulics will place a demand on the hydraulic system pump, in effect drawing power from the loader's internal combustion engine and reducing engine speed. Because of the drag on the engine, the forward power applied to the loader's wheels, known in the art as rim pull, will decrease. Thus, use of the work machine hydraulics will limit the degree to which the bucket is forced into the pile by the work machine's propulsion system.
In electrically propelled work machines, there is no such inherent limitation on the forward propulsion force from the electrical propulsion system. Accordingly, continued forward propulsion of the loader can force the bucket into the pile to such a point that it becomes jammed. The operator may then have difficulty in either lifting the bucket with its contents from the pile, or even backing the work machine out of the pile. The interaction between the work material pile and the hydraulically actuated bucket of the work machine, and the associated hydraulics, is known in the art as “crowd force.”
U.S. Pat. No. 4,776,751 to Saele sets forth one system for controlling crowd force in a hydraulically driven loader. Saele includes a variable displacement hydraulic pump, and a fixed displacement hydraulic motor. Saele utilizes plural hydraulic pressure sensing switches to determine excessive crowd force on the work implement. Upon detecting excessive crowd force, an electronic controller overrides the operator's commands, and decreases fluid output of the hydraulic driving pump. Adjustment of the driving pump will decrease the fluid supply rate/pressure to the hydraulic motor, in turn limiting the forward driving force of the work machine. Saele thus appears to provide one workable approach to crowd control, albeit only for a specific type of loader.
The present disclosure is directed to overcoming one or more of the problems or shortcomings set forth above.